It is a well known characteristic of motor boats that when under power, the bow of the boat tends to rise. As the bow of the boat rises, the angle of the boat changes and the angle at which the prop sits in the water changes, thereby decreasing the efficiency of the motor/prop combination. Ideally, the boat's prop is held horizontally in the water in order to maximize the efficiency of the prop and motor. With the prop held in a horizontal position, all of the propulsive force generated by the prop is directed horizontally in the desired direction. However, as the bow of the boat rises, the boat no longer remains horizontal with respect to the water, and the prop is placed at an angle. As a result, the propulsive force generated by the prop is no longer horizontally directed. Therefore, to maintain the maximum efficiency of the motor/prop, it is important that the prop be held as horizontally as possible. Various systems exist to manually adjust the trim of the boat drive while the boat is under power by means of electric motors or hydraulic actuators. These systems require the boat operator to estimate the desired trim angle and adjust the trim manually by means of the electric motors or hydraulic actuators.
There have been proposals for systems which automatic adjust the trim angle. These proposals are rather complex, usually involving subsystems for measuring the boats speed and micro-processors for calculating the optimum trim angle and controlling the trim motors. While potentially effective, these systems are expensive to implement. A simple system of automatic trim control is therefore required.